1. Field of the Invention.
This invention relates to the field of the preventing excessive accumulation of polymerized hydrocarbon and/or hydrocarbon-silicone films on critical external optical spacecraft surfaces and of cleaning the surfaces from such polymerized films which do accumulate on these surfaces. More particularly, it pertains to the use of photocatalytic coatings, such as titanium dioxide (TiO2), to oxidize the hydrocarbons and to prevent the accumulation of the polymerized films on the protected surfaces.
2. Description of the Related Art.
In space, external optical surfaces such as those of imaging optics, thermal radiators, solar panels, cryogenic infra-red optics, warm visible-light and ultra-violet light optics, on spacecraft in orbits become contaminated as a result of polymerization of hydrocarbons adsorbed on those surfaces. These hydrocarbons are present in vapors that outgas from onboard organic materials such as bonding agents, foams, conformal coatings on circuit boards, potting compounds and the like. After being adsorbed on optical surfaces, these organic materials polymerize, at least on the sun-illuminated surfaces, followed by cross-linking under the harsh conditions of UV exposure. As a result, high molecular weight polymers are formed on the optical surfaces. They form a yellowish deposit that darkens with time.
Such contamination in space is harmful to the smooth operation of a spacecraft and poses a serious technical problem. The functions of thermal radiators and of solar panels are substantially impaired and compromised as a result. The solar panels become seriously degraded with time as the thickness of the polymer films increases and less and less sunlight penetrates to the photovoltaic junctions. The thermal radiators lose their cooling effectiveness because they absorb more and more sunlight and convert it into heat as the films thicken. In addition, reflective or Fresnel-lens solar concentrators, anticipated for use on near-future spacecraft, will also darken and transmit less and less sunlight to the solar panels as the photopolymerized films accumulate. If nothing is done, the on-board electronics will either have to be shut off to avoid overheating or will be shut off because there will be not enough electricity generated.
To neutralize the problem of the polymeric film growth on the critical optical surfaces, the thermal radiators and solar panels should have an overcapacity (and usually it means they must be oversized), and/or deployable radiators must be used (the radiators are mechanically deployed after the original surfaces have become contaminated), or an active cleaning system must be provided. The use of both oversized radiators and/or of deployable radiators adds weight, cost and complexity to the spacecraft. In addition, the use of oversized radiators would require consumption of precious electrical power to heat the spacecraft early in its life, before the films have built up. With the deployment systems there is always an added risk of their failure after years in space. Therefore, the development of a cleaning system remains the most attractive alternative.
A known method of active cleaning of the optical surfaces is a method of plasma cleaning. U.S. Pat. Nos. 5,928,461, 5,628,831, 5,418,431, and 4,977,352 describe this plasma cleaning method which is believed to be a current state-of-the-art of active cleaning methods. This plasma system involves the use of a radio-frequency plasma source to form a plasma of a reactant gas (water) that floods the surfaces to be cleaned. The plasma contains reactive species, i.e., excited neutrals, ions, free radicals, that break the polymeric films into low molecular weight neutral substances that quickly desorb at typical spacecraft temperatures. This method, however, has serious disadvantages. It requires a plasma source, a radio-frequency power supply, control electronics, and reservoir of reactant. All of this makes the spacecraft more expensive, heavier and more complex. There is also a increased need for power to run the radio-frequency power source. For all these technical and economic reasons, this cleaning system is not believed to have been used on an actual spacecraft.
A method for self-cleaning of windshields and bathroom tiles using titanium dioxide-based coatings for photocatalytic oxidation of hydrocarbons has been described in a printed publication. Chemical and Engineering News , Sep. 21, 1998, Pages 70-74. However, it is believed that the method has never been proposed for space applications. There exists no fielded system for cleaning optical surfaces in orbit. Yet, as discussed above, the need for such system is acute.
For the foregoing reasons, there is a necessity for an active system for cleaning crucial optical surfaces of spacecraft in orbit. The system needs to be reliable, effective and must be low in cost.
The present invention is directed to a composition that satisfies the need for a reliable, effective and low cost system that will help ensure that an excessive accumulation of polymerized hydrocarbon and/or hydrocarbon-silicone films on crucial optical surfaces of spacecraft will not occur. It is further directed to the cleaning method that will ensure that even if some polymeric film build up does take place, the surfaces can be easily cleaned while the spacecraft is in its orbit. Alternatively, the present invention would allow cleaning of surfaces of aircraft of similar contamination. As another alternative, bodies of automobiles and windows of buildings can also be coated thus helping to reduce urban hydrocarbon pollution.
A coating in accordance with this invention is applied to the surfaces to be protected. Materials for the coating comprise titanium dioxide (along or being doped with metals like copper or silver), and titanates. The thickness of the coating is preferably within the range of 5 Angstrom to 2xc3x97104 Angstrom and it can be applied using a variety of methods including sputtering, electron beam evaporation, and sol-gel processing.
The gist of a cleaning method in this invention is that the coating catalyzes reaction between the hydrocarbons and oxygen, thus preventing their polymerization on the surfaces and causing their safe evaporation and dissipation into the environment. This can occur without human participation and without expensive mechanical or electrical devices to help the system function. As such, the system can be completely passive and, in the presence of an oxidizing agent, self-cleaning can occur. At orbital altitudes above low-earth orbits (LEO) there is little or no oxygen normally available outside the space craft, so a stream of an oxidizing agent, including, but not limited to, H2O2 or oxygen, is released and directed at the surface, either continuously or periodically. The surfaces at such orbital altitudes are cleaned by a combined action of the coating, the oxidizing agent, and solar UV-light. At LEO altitudes and for spacecraft in Molniya orbits, surfaces that face into the ram direction (usually East) will be self-cleaning due to the oxygen available in the atmosphere; wakeside surfaces may require supplemental oxidizing agent for effective cleaning.
In the case of solar panels and solar concentrators, solar UV is present at all times except during spacecraft eclipse, so cleaning can occur almost all the time, if needed. Radiator panels are commonly fitted to geostationary spacecraft with one panel facing north, and the other facing south. Because of the seasonal motion of the sun, the north radiator will be sunlit only between the vernal and autumbral equinoxes, while the south radiator will be sunlit only during the other half of the year. Since cleaning can only take place in the presence of solar illumination, a useful cleaning strategy may be to clean each radiator just before it passes into darkness. This will keep the radiator clean during its half-year of darkness, since photopolymerization does not occur without sunlight.